Atomized lead-aluminum alloy powder article



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. 3,139,334 ATOMIZED LEAD-ALUMINUM ALLOY POWDER ARTICLE p Fritz VictorLenel, Rensselaer, N.Y., assignor to Lead Industries Association, Inc.,New York, N.Y., a nonprofit corporation of New York No Drawing. FiledSept. 29, 1961, Ser. No. 141,577

1 Claim. (Cl. 75.5)

This invention relates to lead-base alloys having high strength andresistance to creep, and adequately high ductility.

Pure lead is soft and weak; .its rate of creep at room temperature is sohigh that in many applications lead cannot be used as a structuralmaterial in itself. The strength and creep resistance of lead can besomewhat impoved by alloying, particularly ifthe alloying element causesprecipitation hardening as in conventional leadantimony and lead-calciumalloys. t

In precipitation hardening, an alloying element is added to the lead,which has higher solid solubility at temperature near. its melting pointthan at room temperature. The alloy is rapidly cooled from the hightemperature and then aged.

However, the extent to which lead can be strengthened and made morecreep resistant by this type of alloying is limited. For instance, at 86F. antimonial lead .containing 6% Sb, has a creep rate of 1% in one yearat a stress of 400 p.s.i.; and of 100% in one year at a stress of 860{mi An age hardened lead-calcium alloy with 0.03% Ca. has creepresistance similar to the antimonial lead alloys.

In contrast, one of the lead base alloys according to the presentinvention, containing 0.55% aluminum, has, at 77 F., a creep rate ofless than 0.5% in one year at a stress of 2500 p.s.i.; such alloy alsohas adequate tensile properties and ductility. I

The preferred properties of prior art age hardened leadbase alloys areobtainedby choosing a suitable chemical composition for the alloy andthen giving the alloy an appropriate mechanical and thermal treatment.For the alloys according to the invention it is also necessary to choosea suitable chemical composition and appropriate mechanical and thermaltreatments.

Incontrast to most conventional alloys, the alloys according to theinvention are fabricated by atomizing them into a fine powder, thencompacting the powder, and then extruding the powder compact.

Ithas also been previously proposed to strengthen lead by atomizing itinto a lead powder, then compacting it and working the compact. Thedispersion of the lead oxide present in the powder strengthens the leadbut such lead is so brittle as to be impractical for many uses.

It is an object of the invention to provide a lead base alloy articlehaving greater resistance to creep and having also high strength andadequate ductility. Other objects of the invention will be more apparentfrom the following description and claim.

A preferred method of producing the alloy of the invention is describedin the following, for two lead-aluminum alloys containing 0.55 and 1.7weight percent of aluminum respectiv ly, the remainder all lead.

. 0.15 inch in diameter.

The alloy is melted in a suitable crucible and heated to a temperaturehigh enough to dissolve all the aluminum in the liquid lead. Thistemperature is in the range of 1500to 1800 F. The melting point of leadis 621 F. and the melting point of aluminum is 1219.4 F.

The molten alloy is then atomized in the following manner:

A thin stream of the molten alloy is produced by pouring it through asuitable nozzle. This stream of metal isbrokenup into very fine dropletsby impinging a stream of air of suitable velocity upon the stream ofmolten metal. The droplets of molten alloy freeze into powder particles.

The resultant powder is collected in a suitable container and siftedthrough a 325 mesh sieve. Only that portion of the powder, finer than325 mesh (less than '44 microns) is used in the preparation of thealloy.

The powder is compacted in the atomized (as-received) condition. Nocompacting was done on reduced powder since it has been found that .bythe hydrogen-loss test, no readily reducible oxides exist in thesepowders. This is discussed more at length below.

The as-received powder is cold compacted into a compact 1 inch indiameter in a steel die under a pressure of 35,000-40,000. p.s.i. Theresultant compact has a density of 98-99% of the density calculated bythe rule of mixtures for a completely dense 99.45% Pb-0.5S% Al alloy.The compact is extruded by indirect extrusion into a wire 0.156 inch indiameter at a temperature in the range 300 to 450 F.

It should be understood that the above method of consolidating thepowder into a solid shape is only one of several methods which may beused to produce a solid shape. For instance, the lead powder may be fedinto a rolling mill and rolled into a sheet. Cold rolling the powderinto a sheet, and then hot re-rolling the sheet in the same temperaturerange as used for extrusion, results in rolled sheet 'having propertiessimilar to those for the extrusions described below.

The preferred alloy composition is 99.45% lead-0.55% aluminum. Thisalloy composition and other lead-aluminum alloys have been tested in theform of extrusions, The tensile properties and the creep rate has beendetermined for extrusions made from as-received powder at extrusiontemperatures as follows.

The data obtained are shown in Tables I to V below.

TABLE I Mechanical Various Extrusion Temperatures [Characterized bysevere embrittlement] Extrusion Temperature, F

Tensile Percent Strength, Elongation p.s.i. in 1 inch Powder map-m 3TABLE 11 Creep Stresses of Various Rolled Lead Alloys and of 95% Lead-5%Copper Powder Extrusions Compared to 99.45% Lead-0.55% Aluminum PowderExtrusion Test D in Table II refers to copper-lead powder alloys asdisclosed in copending application Ser. No. 99,995, filed April 3, 1961,in the names ofSchrade F. Radtke and Fritz' V. Lenel. g

a I TABLE III Mechanical Properties of As-Received Lead-Aluminum PowderExtrusions With Varying Amounts of- Aluminum l Weight, Extrusion TensilePercent Test Percent Tempera- Strength, Elongation Aluminum ture, F.p.s.i. in 1 inch A 0. 55 450 5, 420 20. o B 1.7 450 3,680 28.0

TABLE 1v 7 Steady-State Creep Rate of the Alloy of Lead With 0.55%Aluminum at 77 F.

[Extrusion temperature 450 F.]

Steady-state Test Stress, p.s.i. Creep rate, Percent/year A 2,000 11 s;than B 2, 500 Lei Ethan o 3,000 1.8 D- I 3,200 3.1

TABLE V Creep Rates of the Alloy of Lead With Steady-State .'1.7%Aluminum at 77 F.

A practical use of this invention is for lead products which requireresistance to creep, either at room or at slightly elevatedtemperatures. For radiation protection, lead maybeextruded into bricksor into other products.

For example, the extruded products may be used in leadsteel. orlead-concrete sandwich structures, which must keep close dimensionaltolerances. The design of these I tron radiation is involved, becausealuminum has a very low capture cross-section for neutrons and istherefore not apt to produce secondary radiation whenirradiated withneutrons.

It is believed that the improved strength and creep The strength andcreep resistance of the alloy will depend upon the degree of dispersionof the aluminum particles in the particles of powder; the finer thedispersion, the stronger and more creep resistant the alloy. The degreeof aluminum dispersion in the particles of powder will in turn depend onthe rate of cooling of the alloy from the temperature where the aluminumis in solution. The finer the particles of powder, the faster the rateof cooling. It is therefore desirable'to use quite n p er- Workhas alsobeen done on powders of ditferent particle size. Even though bestresults have been obtained with powder, all of which is below 44microns, good results have been obtained with powders of larger particlesize.

In' contrast to the lead-copper alloy powders the leadaluminum alloypowders have a very low hydrogen loss. The hydrogen loss is the percentloss in weight of a sample when it is heated in hydrogen at a certaintemperature for a certain time (1020' F. for 30 minutes for lead). It istherefore an indication of the amount of oxide which can be readilyreduced by hydrogen under the given reducing conditions. In lead-copperpowders the hydrogen loss is due to the presence of lead oxide, orpossibly of copperoxide, which can be readily reduced by hydrogen. The

low hydrogen loss in lead-aluminum powders means that oxide. Since A1 0cannot be reduced under the condi- [Extrusion temperature 450 F.]

' Steady-state Test Stress, p.s.i. Creep rate,

Percent/year tions of the hydrogen loss test, the hydrogen loss is low.It is possible that a considerable portion, or possibly even all,'of thealuminum' in the lead-aluminum powder is in the form of aluminum oxide.

It is not inconceivable that the higher creep-resistance of thelead-aluminum powder extrusions, compared with the lead-copper powderextrusions, is due to the fact that aluminum oxide is a more effectivestrengthener. than metallic copper or that the aluminum oxide is morefinely divided in the lead than the metallic coppen. It is also possiblethat the higher strength and'creep resistance of the alloy with 0.55% Alcompared with those of the 1.7% A1 'is due to the form in which-thealuminum is which the aluminum is present'tinetallic' aluminum vs;

aluminum oxide) or due to the distribution of thealuminum or'its oxide(coarse vs. fine).

Work has also been done to determine the range of composition of thePb-Al alloys, which have these desirable properties. This Work indicatesthat. the aluminum content should-run from 0.1% to 2.0% aluminum byweight, remainder lead. I

What is claimed ist' 'A worked lead base powder article, the particlesof said powder being obtained by atomizing ,a molten leadaluminum alloyin whichthe. aluminum is completely dis solved in the lead, saidparticles consisting of a fine dispersion in a lead matrix of a form ofaluminum selected from the group consisting of aluminum and aluminumoxide, and said lead-aluminum alloy, before atomizing,

consisting essentially of from 0.1 (02.0% by weight of resultingcompacted form, said article having a substantially lower creep ratethan that of pure lead.

References Cited in the file of this patent UNITED STATES PATENTSWilliams July 12, 1927 Williams June 3, 1930 Cooper May 2, 1939 McKimSept. 11, 1962 Giraitis et al Apr. 16, 1963 6 FOREIGN PATENTS GreatBritain Oct. 7, 1959 OTHER REFERENCES Campbell et al.: The Alloys ofAluminum and Lead, Canadian Journal of Research, vol. 18, sec. B. 1940,pages 281-287.

Claus et al.: Zur Kenntnis der Systeme AlPbCu und Al-Pb,Metallwirtschaft, 18, 1939,. pages 957-960.

Hansen: Constitution of Binary Alloys, 2nd ed., 1958, McGraw-Hill BookCompany, Inc., N.Y., pages 122-124.

